Keeping your common areas clean is something that most residents will expect from you if they pay rent. If you go into a hotel, you expect their lobby to be reasonably clean along with any hallways and rooms such as laundry areas or vending spaces. While your apartment building is slightly different due to your residence, the standards to which many residents hold do not differ all that much. Making sure that the floors and walls and even ceilings are kept appropriately clean is something that you should put at the top of your list. Having a small cleaning crew may seem costly, but there are certainly ways around this. You may ask around to see if any of your residents are looking for a part time job, especially if there are teenagers or older tenants. You can always offer to either pay them slightly less than others may take or you can offer to take some money off of their rent in exchange for the work.
Having a type of security can always help to make your tenants happier as well. This can be with anything from motion sensor lights to having an actual security system installed. Having a buzzer or telephone entry system normally won't break the bank, or you can always hire someone to work security for the building. Ask your tenants what would make them feel safer and see what their response is.
If you want to keep good tenants, you need to keep them happy. The easiest way to do that is to ask them what they would like to see the most. See what they say and if possible, make it happen. It doesn't need to be difficult, which is the beauty of helping your tenants out.